1. Field of the Invention.
The present invention pertains to the sensing of color and more particularly to an optical arrangement whereby the color of a surface is sensed utilizing light reflected from the surface and directed to at least two detectors characterized to respond to different wavelength bands. The output of the detectors is analyzed to determine the relative amounts of radiation in the bands as an indication of the color of the surface.
2. Description of the Prior Art.
Systems for sensing color have been devised in the past. One such system involves directing light through a spherical lens at an angle to the surface to be sensed and focussing the reflected light through filters that are arranged to pass only the primary colors, onto light responsive detectors to produce output signals intended to be indicative of the amount of each primary color present in the reflected energy.
Another such system, rather than using three separate filter-detector combinations, uses a single detector and a plurality of filters movable, one at a time, in the path of reflected light from the surface to the detector. The series of signals from the detector are analyzed to determine the color of the surface.
Several difficulties have been encountered with prior art systems such as described above. For example, these systems utilize spherical lenses to direct the beam of light onto the surface and focus it on the detectors. Spherical lenses have fairly large f numbers, typically greater than f1.4, and accordingly, the flux density of the light on the surface being investigated is limited. This is because the flux density depends on the area of the lens divided by the square of the distance to the surface and is inversely proportional to the square of the f member. Thus, the flux density decreases with the square of the f number making it undesirable to use lenses with large f numbers. For best results, it would be desirable to employ f numbers below 1 to obtain greater amounts of usable light but with spherical lenses this is not practically possible. Furthermore, when the light reflected from the surface is focussed onto the three detectors, as in the first system described above, each detector receives energy from a different area of the reflected beam, and each different area may have different amounts of the primary colors used to determine the color of the surface. Thus the reliability of such systems is poor. Furthermore, the filters and detectors when placed in the beam, in addition to receiving different amounts of primary colors are usually at different distances from the surface and accordingly receive different amounts of flux. Thus this system is, at best, a rather inaccurate representation of the amounts of primary colors in the surface being sensed and the resultant output is often in error. The alternate system described above is very slow in operation since the filters have to be moved into the reflected light path one at a time. Accordingly the alternate is unsatisfactory when color analysis is desired quickly as, for example, on a production line with products being moved rapidly past the color detector apparatus.